This invention relates to the field of respiratory care, and more particularly to the care of ventilator dependent patients who require oxygen enriched breathing gas to properly maintain ventilation.
Patients suffering from neuromuscular disease, spinal cord injuries, or various chronic respiratory diseases are often unable to breathe by themselves. Typically, these patients are intubated, which refers to the placement of a tube in the trachea to provide a sealed pneumatic conduit to the lungs. Along with the intubation, the patients are typically placed on a positive pressure ventilator. A typical ventilator system would include a blender to mix incoming air and oxygen to a desired concentration, an inspiratory flow control system to control the flow of gases to the patient's lungs, and an exhalation system to control the flow of gases from the lungs.
Ventilators for hospital use are designed to receive compressed air and oxygen from the hospital gas supply system which typically provides these gases at a pressure of about 50 psig. The gas blending and flow control systems of these ventilators are designed to work with these pressurized gas sources.
In recent years an increasing number of ventilator dependent patients are being cared for in the home. This places new design constraints on ventilator systems. Compressed air is typically not available in the home as it is in hospitals. Consequently, the ventilator blending and flow control systems for home use must be designed to operate using air at ambient pressure.
The delivery of oxygen enriched gas for ventilator dependent patients is currently implemented in one of two ways. In the first type of gas delivery system, a continuous flow of 100% oxygen from a pressurized cylinder is metered through a flow valve and meter. This flow is injected directly into the patient circuit. The proper flow rate is calculated for each ventilation condition to obtain the desired oxygen concentration from equations known in the art.
As the calculation requires knowing the minute volume and the desired oxygen concentration, this method has the disadvantage that the minute ventilation of the patient must be known. For patients who breathe spontaneously, the minute ventilation varies based on the patient's breath activity. Consequently, the oxygen concentration varies along with the minute ventilation. This method also requires that the proper flow rate be recalculated each time the ventilator settings are changed. Thus, the true oxygen concentration is never displayed. A further disadvantage is that during exhalation, the continuous flow of oxygen fills the patient circuit. When inspiration occurs, the patient receives 100% oxygen for the first portion of the breath followed by nearly pure air for the remainder of the breath. The result is delivery of wildly varying oxygen concentrations within a single breath.
A second type of gas delivery system uses a mixing box fluidly attached to the ventilator inlet, where a continuous flow of oxygen is injected into the box. The home ventilator draws in mixed air and oxygen from the box for delivery to the patient. This technique eliminates the mixing problem where at first only oxygen, and then only air is delivered to the patient. But this technique is still subject to the problems described above relating to the oxygen variation with minute ventilation, and the repeated recalculation of the flow rate for any change in ventilation.
There is thus a need for a gas delivery system for home ventilation use which does not vary the oxygen concentration with the minute ventilation of the patient. There is a further need for a gas delivery system which does not require continuous recalculation of a gas flow rate. There is a further need for a gas delivery system which delivers a substantially uniform mixture of gases. There is a further need for an ability to visually and accurately display the oxygen concentration over the operating range of the system.
A further difficulty with home ventilator systems is the broad range of operating requirements which are desirable. Current gas blending systems used in hospitals cannot be applied directly to home ventilator use because the devices are designed for use with the pressurized gas sources, over relatively small ranges of operational requirements. A typical blender designed for hospital use has a balance mechanism to substantially balance the pressure of the pressurized gases, and a proportioning mechanism to mix or blend predetermined portions of the gases after their pressures have been substantially balanced.
For example, if the air input pressure was 3500 cmH.sub.2 O (50 psi), and the oxygen inlet pressure was 2800 cmH.sub.2 O (40 psi), the pressures of air and oxygen exiting the balance system would typically be 2801 cmH.sub.2 O and 2800 cmH.sub.2 O respectively. It is important to note that the balance system does not yield exactly balanced pressures. One such pressure balance mechanism is described in U.S. Pat. No. 3,727,627 to Forrest Bird.
The two nearly balanced gases then enter the proportioning subsystem, the heart of which is typically a double ended poppet operating between an oxygen seat and an air seat. The poppet is positioned between the two seats by a knob/screw actuator to achieve the desired oxygen concentration. As the flow of one gas increases, the flow of the other gas decreases. With the control knob at the 21% position, the poppet is seated on the oxygen seat, allowing only air to flow. At the 100% position, the poppet is seated on the air seat, allowing pure oxygen to flow. Intermediate positions of the poppet yield oxygen concentrations between 21 and 100%.
A typical specification for such a hospital gas blender would be +/-3% accuracy over a flow range of 12 to 120 liters per minute (lpm), with inlet pressures of about 50 psig. That amounts to a flow ratio of 10:1. The minimum flow through the blender is determined by the permissible pressure drop across the proportioning valve and by the balance capabilities of the balance modules. The pressure drop across the proportioning valve is governed by the follow general equation: EQU Pressure Drop=K*A*Q.sup.2
where: K=constant related to the gas
A=valve area PA1 Q=gas flow rate
Due to the squared relationship, it can be seen that for a 10:1 ratio in the gas flow rate, the corresponding pressure drop would be (10).sup.2 :1 or 100:1. As discussed below, these operational capabilities are inadequate for home use.
Home care ventilators typically use one of two available drive systems to actuate a pumping piston which provides the gas pressure to overcome pulmonary resistance and to control the flow rate of the gas into the lungs. The first and simplest drive is a crankshaft type mechanism not unlike that used on a typical automotive engine. The second type of drive uses a direct current (DC) motor coupled to the piston through a ball screw arrangement. The intake flow profiles of these systems vary widely depending on ventilator settings and the drive profile used by the designers of each particular device. Based on selective testing, it is believed that a flow range of 5 to 160 lpm is needed to satisfy the requirements of the various existing ventilators designed for home use.
Since the ventilator must provide the motive force for drawing the blended gases in through the blender, the blender must have a low pressure drop to avoid slowing down the ventilator intake stroke and/or causing excessive energy consumption. It is believed that the pressure drop must not exceed 35 cmH.sub.2 O through the specified flow range, based on limited empirical testing.
The performance specifications for a home use blender capable of being used with the current spectrum of home ventilators would thus have flow range of 5 to 160 lpm, a maximum pressure drop of 35 cmH.sub.2 O, a blending range of 21 to 50% of oxygen, and an accuracy of +/-3%. While the air is supplied from the atmosphere at ambient pressure, the oxygen must come from a pressurized source such as a tank or an oxygen concentrator. The pressurized oxygen must be depressurized to ambient pressure using a pressure reducing valve or a throttling orifice feeding a rubber bag. In either case, the pressure of the oxygen can be expected to be 0 to 1 cmH.sup.2 O above ambient when measured at the proportioning device 12.
To maintain the +/-3% accuracy specification with the 0 to 1 cmH.sub.2 O oxygen inlet pressure variation from the balance regulator, the minimum pressure drop at 5 lpm must be approximately 5 cmH.sub.2 O. Using conventional blender technology as previously described, the associated pressure drop at 160 lpm would be (32).sup.2 *5, or 5,120 cmH.sub.2 O.
As can be seen, both the flow and pressure ranges required for the ambient air blender are beyond the capabilities of conventional oxygen blender technology. The ambient pressure blender must be capable of a flow range of 5 to 160 lpm (1:32 ratio), while current blenders have a flow range of 12 to 120 lpm (1:10 ratio). Moreover, the ambient pressure blender must maintain a maximum pressure drop of 35 cmH.sub.2 O or a maximum pressure ratio of 7:1, while current blenders have a maximum pressure drop of 3500 cmH.sub.2 O or a maximum pressure ratio of 700:1.
There is thus a need for a home ambient gas blender having greater operational capabilities than previously available. There is a further need for such a system to be small, of simple construction, and reliable, in order to facilitate its use in homes where trained service personnel are not available to continuously monitor the performance of the machine, to adjust it, or to repair it.